This invention generally relates to the measurement of blood flow and more particularly to the measurment of blood flow from the heart in a patient being aided by an intra-aortic balloon pumping system.
An intra-aortic balloon pumping system is used in the treatment of ischemic heart disease. The system includes a long slender balloon which is inserted surgically into a patient's descending thoracic aorta. An external system controller inflates and deflates the balloon in synchronism with the patient's heartbeat. The alternate inflation and deflation of the balloon boosts aortic diastolic pressure, lowers systolic pressure, and increases coronary blood flow. As a result, the balloon pumping system increases or maintains cardiac performance while reducing the workload on the patient's heart.
The monitor functions in response to the signals normally present in an intra-aortic pumping system: a pressure signal from a catheter in the radial artery and associated transducer, a balloon pressure signal representing the pressure within the balloon, and a balloon drive signal which controls the inflation or deflation of the balloon.
It is, of course, extremely desirable to measure the effectiveness of the balloon pumping system treatment. A key parameter in this regard is cardiac output, which is the total blood flow rate from the heart. Diverse, clinically acceptable methods for measuring cardiac output are used and other methods have been proposed.
The oldest and most reliable method is known as the Fick method. This method was first proposed in the late 1800's, but did not gain acceptance as a clinical method until procedures for cardiac catheterization were perfected. In accordance with this method, a catheter is implanted in the pulmonary artery to measure the oxygen concentration in venous blood while another catheter is implanted in the brachial, radial or femoral artery to measure the oxygen concentration in arterial blood. Patient oxygen comsumption is determined by breath analysis. The ratio of the differential of the arterial and venous blood oxygen concentrations to the patient oxygen consumption indicates cardiac output.
As can be appreciated, the Fick process is quite complicated and requires expensive apparatus. Even though it is recognized as being reliable, a dye method for determining cardiac output is now more popular. In accordance with this method, known since the late 1940's, a dye solution is injected through a catheter into the pulmonary artery. A constant flow blood sample is then withdrawn from another artery to provide a time analysis of dye concentration which leads to a determination of cardiac output. This method, although widely used, has two disadvantages. It is less accurate than the Fick method. Also, a patient may have an allergic reaction to the dye solution, so the method can not be used universally.
Another clinical method which has been used since the introduction of thermistors in the 1950's is a thermal dilution method. With this method, a cold saline solution is injected through a catheter into the right atrium. The dispersion of that solution is monitored by inserting a catheter with a thermister into the pulmonary artery. Cardiac output can then be determined by analyzing the temperature variations.
Other methods are experimental. In one such method, a magnetic flow meter is implanted in the ascending aorta. Leads from the flow meter are then connected to an external controller which indicates the blood flow at the flow meter. With still another experimental technique, a catheter implanted in the ascending aorta has means for obtaining spaced lateral pressure taps. The differential pressures from these taps indicate cardiac output.
All these foregoing clinical and experimental methods are invasive. Impedance plethysmography is an experimental non-invasive method. Energizing electrodes disposed on the skin are excited by an AC signal. Sensing electrodes, also on the skin, receive the signals and couple them to a controller which analyzes the exciting and receive signals for displaying cardiac output. Although this technique is non-invasive, the patient is subjected to electrical signals which, in some cases, may have deleterious effects.
When a patient is undergoing treatment by means of an intra-aortic balloon pumping system, all the foregoing methods for determining cardiac output require additional external or internal connections to be made to the patient. These connections can complicate treatment. None of the methods, other than the Fick method, produces an extremely accurate indication of cardiac output. As known, they are subject to rather substantial errors. Moreover, none of the clinical methods permit or facilitate continuous monitoring of cardiac output.
Therefore, it is an object of this invention to provide an improved method and apparatus for measuring cardiac output in a patient undergoing treatment by means of an intra-aortic balloon pumping system.
Another object of this invention is to measure cardiac output by using signals which are presently available in an intra-aortic balloon pumping system.
Still another object of this invention is to provide apparatus for measuring cardiac output continuously.
Yet another object of this invention is to provide apparatus for measuring cardiac output in an intra-aortic balloon pumping system without further invasion into the patient.
Still yet another object of this invention is to provide apparatus for measuring cardiac output which facilitates measurements and recording at the time the measurements are made.